1) Field of the Invention
This invention relates to a process for removing impurities from crude methacrylic acid obtained as an aqueous solution by vapor-phase catalytic oxidation of isobutylene or the like.
2) Description of the Prior Art
Regarding the production of methacrylic acid by catalytic oxidation of isobutylene, tertiary butanol, methacrolein or isobutyl aldehyde with molecular oxygen in the presence of steam in accordance with a single-stage or two-stage reaction, a number of proposals have been made including, for example, catalytic reactions disclosed in U.S. Pat. Nos. 4,001,317, 4,301,031 and so on, purification and recovery methods and processes disclosed in various publications, etc. A reaction gas obtained in such a manner contains byproducts in addition to methacrylic acid as the target product, for example, carboxylic acids such as formic acid, acetic acid, propionic acid, maleic acid, citraconic acid, benzoic acid, toluic acid and terephthalic acid, and aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, methacrolein, benzaldehyde, tolualdehyde and furfural. Most of these impurities can be removed by a conventional purification method such as extraction or distillation. It is however difficult to remove impurities which are contained in trace amounts. For example, it is difficult to remove maleic acid, citraconic acid and aldehydes completely. When aldehydes are contained in particular, absorption is observed in the ultraviolet range, and inconvenient problems arise in many instances such that at the time of a polymerization reaction, the polymerization is suppressed to require longer reaction time and the resulting polymer is tinged.
As a method for removing aldehydes from methacrylic acid in which the aldehydes are contained, it has been known to add an amine such as hydrazine, ethylenediamine, aniline or polyamine (Japanese Patent Laid-Open No. 23017/1977), ethylene glycol (Japanese Patent Laid-Open No. 128336/1983), a bisulfite (Japanese Patent Laid-Open No. 252446/1985), a mercaptan (Japanese Patent Laid-Open No. 6635/1985), resorcin, pyrogallol or a-naphthol (Japanese Patent Laid-Open No. 130546/1985), or the like.
However, the use of the above-described amine cannot bring about sufficient effects for the removal of aldehydes and moreover, tends to induce polymerization in a distillation step to be performed after the treatment. Glycols, bisulfites and mercaptans cannot exhibit strong effects for the removal and must hence be added in a large amount, whereby such additives cause secondary contamination or reaction loss of methacrylic acid. On the other hand, phenols such as resorcin can exhibit removal effects only in the co-presence of a strongly acidic substance such as sulfuric acid or hydrochloric acid. Accordingly, they render the operation complex and requires the selection of high-quality materials for actual facilities.